My plan is to get a decent sum saved up in accounts for each and have that be the only monetary support they get from us. Let them know it's there and available, but that everything above and beyond that will be on them to pay for.

Hopefully, they can cover enough of their schooling through their own funds from working, scholarships, etc. to not need all of what's saved in the 529 and can roll a decent chunk of it into a Roth account when they finish up with school.